1. Field of the Invention
The invention generally relates to a power conversion driving circuit, and more particularly, to a power conversion driving circuit having the function of automatically turning off and re-starting.
2. Description of Related Art
Current power supplies are mainly classified into linear power supplies (LPS) and switching power supplies (SPS). The LPS has a simple circuit, small ripples, and less electro-magnetic interference (EMI). However, electric devices in the circuit are large, so that the volume of the circuit is large, and the weight thereof is heavy, and further, conversion efficiency thereof is low. On the contrary, even though the SPS has a more complex circuit, larger ripples, and more EMI, the SPS is still mainstream in the market of power supplies since it has higher conversion efficiency and less power consumption while idling.
FIG. 1A is a schematic circuit of an SPS configured to drive a lamp in a related art. Referring to FIG. 1A, The SPS includes an initial resistor R, an initial capacitor C2, a Zener diode Z, a controller CON, a high-side driving capacitor C1, a high-side driving transformer T1, a high-side transistor switch M1, a low-side transistor switch M2, a diode D, an output capacitor C3, and a transformer T2. The SPS is configured to convert a DC input voltage VIN to an AC output voltage VOUT to drive a lamp LAMP.
When the DC input voltage is inputted, a current is supplied to the initial capacitor C2 through the initial resistor R, so that a voltage drop across the initial capacitor C2 is raised until it is equal to the breakdown voltage of the Zener Diode Z. The initial capacitor C2 generates a driving voltage VDD to supply the electric power required for operating to the controller CON. When the driving voltage VDD is higher than a start voltage of the controller CON, the controller CON is started, so as to generate control signals to control the high-side transistor switch M1 and the low-side transistor switch M2. The controller CON raises a voltage level of the control signal up to a suitable voltage level to control the high-side transistor switch M1 through the high-side driving capacitor C1 and the high-side driving transformer T1. By switching the high-side transistor switch M1 and the low-side transistor switch M2, the electric power of the DC input voltage VIN is transmitted to an output terminal to generate the AC output voltage VOUT to drive the lamp LAMP. The transformer T2 is coupled to the AC output voltage VOUT, and transmits electric power, rectified by the diode D, to the initial capacitor C2.
The initial capacitor C2 gradually stores the electric power due to the fact that the electric power transmitted through the initial resistor R is more than the electric power consumed by the controller CON before the controller CON is started. After the controller CON is started, the electric power through the transformer T2 and the diode D is also supplied to the controller CON. Accordingly, the initial resistor R having a relatively large resistance is used to lower power consumption by the initial resistor R. However, when an abnormal event occurs in the circuit, no more electric power from the DC input voltage VIN is supplied to the to the AC output voltage VOUT, so that the transformer T2 and the diode D can not supply the electric power any more. Moreover, the electric power transmitted through the initial resistor R is not enough to provide all of the electric power required by the controller CON while normally operating, so that the operation of the controller CON may fail.
FIG. 1B illustrates a schematic signal waveform of the SPS configured to drive the lamp in the related art while the circuit stays in the abnormal state. Referring to FIG. 1B, when the driving voltage VDD is higher than the start voltage UVLO, the SPS starts to operate. At this time, since an oscillator and a control circuit inside the controller CON have started to operate, the current consumed thereby is much more than the current supplied by the DC input voltage VIN through the resistor R. Accordingly, the driving voltage may start to fall down. When the circuit operates at a normal state, the controller CON outputs signals to switch the high-side transistor switch M1 and the low-side transistor switch M2, so that the AC output voltage VOUT is raised, and the electric power is supplied to the initial capacitor C2 through the transformer T2 and the diode D. However, when an abnormal event occurs in the circuit, the controller CON stops switching the high-side transistor switch M1 and the low-side transistor switch M2, so that the AC output voltage VOUT is lowered and can not supply the electric power to the driving voltage VDD. As a result, the driving voltage VDD still falls down. When the driving voltage VDD has become lower than a voltage range which the controller CON can operate, the controller CON stops operating and further decreases the consuming power. Accordingly, the driving voltage VDD is raised again until it is higher than the start voltage UVLO, so that the controller CON is re-started. The above-described cycle is repeated until the abnormal event is eliminated. Furthermore, in order to avoid an erroneous judgment that the lamp does not light due to a temporary abnormal event, the controller CON may try to strike the lamp continuously when the lamp does not light in the related art. In the process, not only is life-span of the lamp shortened due to limitation of start cycles thereof, but also users may get an electric shock during lamp replacing if the users forget to turn off the power source. Moreover, if the users turn off the power source first, and next turn on the power source after the lamp has been replaced with new one, the users may not get the electric shock during lamp replacing, but it is not convenient for the users and is different from the normal users' habits.
Accordingly, even though the lamp driving circuit may re-start the lamp in the SPS of the related art, not only is the life-span of the lamp shortened, but also using it may be dangerous to the users.